my powerbook won't power on (no bong or fan )

hello fellow apple friends, i got a broken powerbook from one of my friends (powerbook g4 aluminum 12'' 1.5 ghz)because the ribbon cable connecting the display and the motherboard was broken. so i replaced that and it powered on, but when i was putting it back together it didnt turn on after that. i soon replaced the dc-to-dc board after that. there is a small battery on the machine like a watch battery, i think thats the problem. please help!!!